U.S. patent number 6,882,468 [Application Number 10/663,829] was granted by the patent office on 2005-04-19 for raman amplifier.
This patent grant is currently assigned to The Furukawa Electric Co., Ltd.. Invention is credited to Yoshihiro Emori, Shu Namiki.
United States Patent |
6,882,468 |
Emori , et al. |
April 19, 2005 |
Raman amplifier
Abstract
In a Raman amplifier using three or more pumping wavelengths,
when the pumping wavelengths are divided into a short wavelength
side group and a long wavelength side group at the boundary of the
pumping wavelength having the longest interval between the adjacent
wavelengths, the short wavelength side group includes two or more
pumping wavelengths having intervals therebetween which are
substantially equidistant, and the long wavelength side group is
constituted by two or less pumping wavelengths. When a certain
pumping wavelength is defined as a first channel and pumping
wavelengths which are spaced apart from each other by about 1 THz
from the certain pumping wavelength toward a long wavelength side
are defined as second to n-th channels, respectively, pump lights
having wavelengths corresponding to the first to n-th channels are
multiplexed, and pump light having a wavelength spaced apart from
the n-th channel by 2 THz or more toward the long wavelength side
is further multiplexed with the said multiplexed pump light, and
resultant pump light is used as pump source. Pump lights of all of
the wavelengths corresponding to the channels other than (n-1)th
and (n-2)th channels are multiplexed with each other, and resultant
pump light is used as pump source. Pump lights of all of the
wavelengths corresponding to the channels other than (n-2)th and
(n-3)th channels are multiplexed with each other, and resultant
pump light is used as pump source.
Inventors: |
Emori; Yoshihiro (Tokyo,
JP), Namiki; Shu (Tokyo, JP) |
Assignee: |
The Furukawa Electric Co., Ltd.
(Tokyo, JP)
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Family
ID: |
26583551 |
Appl.
No.: |
10/663,829 |
Filed: |
September 17, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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950779 |
Sep 13, 2001 |
6825972 |
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PCTJP0100094 |
Jan 11, 2001 |
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Foreign Application Priority Data
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Jan 14, 2000 [JP] |
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2000-6567 |
Jun 30, 2000 [JP] |
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2000-199548 |
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Current U.S.
Class: |
359/334;
359/341.3 |
Current CPC
Class: |
H01S
3/302 (20130101); H01S 3/1001 (20190801); H04B
10/2916 (20130101); H01S 3/0677 (20130101); H01S
3/06766 (20130101); H01S 3/09408 (20130101); H01S
2301/04 (20130101); H01S 3/094096 (20130101) |
Current International
Class: |
H04B
10/17 (20060101); H01S 3/30 (20060101); H01S
3/094 (20060101); H01S 3/23 (20060101); H01S
003/00 () |
Field of
Search: |
;359/334,341.3 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 139 081 |
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May 1985 |
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EP |
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0 933 894 |
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Aug 1999 |
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EP |
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10-73852 |
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Mar 1998 |
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JP |
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2001-7768 |
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Jan 2001 |
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JP |
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2001-33838 |
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Feb 2001 |
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JP |
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Other References
Y Emori, et al., Technical Research Report, The Institute of
Electronics, Information and Communications Engineers, vol. 99, no.
52, pp. 25-29, "Hachou Taju Reiki Ld Unit Wo Mochita Raman
Zoufukuki Ni Okeru Hachou Reyouiki 100nm Ni Wataru Ritoku Heitanka"
(100nm Bandwidth Gain-Equalization on Raman Amplifiers Pumped by
Multi-Channel WDM Laser Diodes), May 14, 1999 (with English
Abstract). .
Sugizaki et al. (OFC/IOOC '99 Tech. Digest. Feb. 21-26, 1999).
.
Emori et al. (OFC/IOOC '99 Tech. Digest. Feb. 21-26,
1999)..
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Primary Examiner: Hellner; Mark
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier
& Neustadt, P.C.
Claims
What is claimed is:
1. A Raman amplifier, comprising: a first pump source configured to
provide pump light at a first central wavelength; a second pump
source configured to provide pump light at a second central
wavelength, light from said first pump source and said second pump
source configured to produce a Raman gain having a predetermined
amplification bandwidth in a gain medium; a third pump source
configured to provide pump light at a third central wavelength that
is between said first and second central wavelengths; and a fourth
pump source configured to provide pump light at a fourth central
wavelength, outside of a wavelength range defined by said first and
second central wavelengths, wherein the predetermined amplification
bandwidth is increased when pump lights from said third and fourth
pump sources are applied to the gain medium.
2. A Raman amplifier, comprising: a first pump source configured to
provide pump light at a first central wavelength; a second pump
source configured to provide pump light at a second central
wavelength, light from said first pump source and said second pump
source being applied to a gain medium to produce a Raman gain
having a predetermined gain deviation within a predetermined
amplification bandwidth; a third pump source configured to provide
pump light at a third central wavelength between said first and
second central wavelengths; and a fourth pump source configured to
produce pump light at a fourth central wavelength, outside of a
wavelength range defined by said first and second central
wavelengths, wherein an expanded amplification bandwidth is formed
when pump light from the third and fourth pump sources is also
applied to the gain medium while not substantially exceeding said
predetermined gain deviation in said predetermined amplification
bandwidth.
3. A Raman amplifier, comprising: a first pump source configured to
provide pump light at a first central wavelength; a second pump
source configured to provide pump light at a second central
wavelength, light from said first pump source and said second pump
source configured to produce a Raman gain having a predetermined
amplification bandwidth in a gain medium; a third pump source
configured to provide pump light at a third central wavelength
between said first and second central wavelengths such that the
first, second and third central wavelengths are substantially
equidistant apart; and a fourth pump source configured to provide
pump light at a fourth central wavelength, outside of a wavelength
range defined by said first and second wavelengths, wherein the
predetermined amplification bandwidth is increased when pump lights
from said third and fourth pump sources are applied to the gain
medium.
4. A Raman amplifier, comprising: a first pump source configured to
provide pump light at a first central wavelength; a second pump
source configured to provide pump light at a second central
wavelength, light from said first pump source and said second pump
source configured to produce a Raman gain at C-band in a gain
medium; a third pump source configured to provide pump light to the
gain medium at a third central wavelength; a fourth pump source
configured to provide a pump light to the gain medium at a fourth
central wavelength, light from said third pump source and said
fourth pump source configured to produce Raman gain at L-band in
the gain medium; and a fifth pump source configured to provide pump
light at a central wavelength that is between the first and second
central wavelengths, wherein the C-band and the L-band are
simultaneously amplified in the gain medium.
5. A Raman amplifier, comprising: a first pump source configured to
provide pump light at a first central wavelength; a second pump
source configured to provide pump light at a second central
wavelength, light from said first pump source and said second pump
source configured to produce a Raman gain having a predetermined
gain deviation within C-band; a third pump source configured to
provide pump light to the gain medium at a third central
wavelength; a fourth pump source configured to provide a pump light
to the gain medium at a fourth central wavelength, light from said
third pump source and said fourth pump source configured to produce
a Raman gain at L-band in the gain medium; and a fifth pump source
configured to provide pump light to the gain medium at a fifth
central wavelength that is between the first and second central
wavelengths, wherein the C-band and L-band are simultaneously
amplified and an expanded amplification bandwidth is formed when
pump light from the third, fourth and fifth pump sources is also
applied to the gain medium while not substantially exceeding said
predetermined gain deviation in the C-band.
6. A Raman amplifier, comprising: a first pump source configured to
provide pump light to a gain medium at a first central wavelength;
a second pump source configured to provide pump light to the gain
medium at a second central wavelength, light from said first pump
source and said second pump source configured to produce a Raman
gain at C-band in the gain medium; a third pump source configured
to provide pump light to the gain medium at a third central
wavelength; a fourth pump source configured to provide a pump light
to the gain medium at a fourth central wavelength, light from said
third pump source and said fourth pump source configured to produce
a Raman gain in the gain medium at L-band; and a fifth pump light
configured to provide pump light to the gain medium at a fifth
central wavelength that is between the first and second central
wavelengths such that the first, second and fifth central
wavelengths are substantially equidistant apart in wavelength,
wherein the C-band and L-band are simultaneously amplified in the
gain medium.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an optical amplifier,
and, more particularly, it relates to a Raman amplifier.
2. Description of the Related Art
In wavelength division multiplexed (WDM) systems used in current
optical communication systems, as methods for increasing
transmission capacity, there are a method for expanding a signal
wavelength band to increase the number of multiplexed wavelengths,
and a method for enhancing a transmission rate (bit rate) per
wavelength. In normal WDM systems, due to limitation in gain
wavelength band of an Erbium doped fiber amplifier (EDFA), most
generally, a wavelength for optical signal is selected from a
wavelength band (called as a C-band) of about 1530 to 1565 nm. On
the other hand, since optical amplification can be effected by the
EDFA also in a wavelength band (called as an L-band) of about 1575
to 1610 nm, recently, WDM systems regarding this band has been
developed.
By expanding the wavelength band as mentioned above, capacity which
can be transmitted by a single WDM system can be increased.
Regarding the WDM systems, since a C-band WDM system has firstly
been developed, in order to increase the transmission capacity of
the WDM system, it is desirable that the existing equipment for the
C-band WDM system is utilized and an L-band WDM system is added to
the equipment to gradually increase the capacity. In the
conventional WDM systems using the EDFA, the transmission rate per
wavelength has been enhanced (speeded up) by technically improving
various elements constituting a transmission system. However, in
systems utilizing a discrete amplifier such as EDFA, achievement of
higher speed has been approaching to its limit. In order to achieve
further high speed and/or longer distance transmission, it is said
that incorporation of a distributed amplifier such as a Raman
amplifier into the system is inevitable, and, to this end, various
developments have been made vigorously to permit practical use.
As shown in FIG. 20, the Raman amplifier comprises an optical fiber
as an amplifying medium, and a pump source for generating
stimulated Raman scattering in the fiber. When silica-based optical
fiber is used as the amplifying medium, peak of gain is generated
at a longer wavelength side than a wavelength of the pump light,
i.e., in a frequency band smaller than that of the pump light by
about 13.2 THz. For example, since a difference in wavelength
between 1450 nm and 1550 nm corresponds to 13.2 THz, a pump light
having a wavelength of about 1450 nm is used in order to amplify
the C-band, and a pump light having a wavelength of about 1490 nm
is used in order to amplify the L-band (FIG. 21).
However, single wavelength pumped Raman gain has great wavelength
dependency, and as apparent from FIG. 22, from when the Raman gain
exceeds about 5 dB, it is impossible to suppress gain deviation
below 1 dB regarding an operating band width of 30 nm. In order to
solve this problem, it is effective that a plurality of pump lights
having proper wavelength interval are applied to the Raman
amplifier (i.e., the amplifier is pumped by a multi-wavelength pump
source). According to this method, Raman amplification having
better gain flatness can be achieved in a wider band than the
conventional band. As disclosed in Japanese Patent Publication No.
7-99787 (1995) (particularly, in FIG. 4 thereof), such concept
itself can be understood intuitively. Japanese Patent Application
Nos. 10-208450 (1998) and 11-34833 (1999) refer concrete values of
the wavelength interval and assert that the proper value is 6 nm to
35 nm.
FIGS. 23A and 23B show examples of Raman gain profiles obtained
when the pump light intervals are selected to 4.5 THz and 5 THz,
respectively and DSF is used as an amplifying fiber. As apparent
from FIGS. 23A and 23B, when the pump light interval is increased,
a valley of gain is deepened and gain deviation is increased. In
FIG. 23A, values shown in the following Table 1 were used as
frequency (wavelength) of the pump light and, in FIG. 23B, values
shown in the following Table 2 were used as frequency (wavelength)
of the pump light. In this case, the pump light interval of 4.5 THz
corresponds to 33 nm and the pump light interval of 5 THz
corresponds to 36.6 nm. That is to say, these examples show the
fact that the gain flatness is not so good if the pump light
interval becomes more than 35 nm.
TABLE 1 Pumping frequency Pumping wavelength Wavelength interval
THz nm nm 204.5 1466.0 33.0 200.0 1499.0
TABLE 2 Pumping frequency Pumping wavelength Wavelength interval
THz nm nm 205.0 1462.4 36.6 200.0 1499.0
FIG. 24 shows a gain profile obtained when the pump light interval
is selected to 4.5 THz and three wavelengths are used. From FIG.
24, it can be seen that, when the third pumping wavelength is
added, the valley of gain is deepened in case of the pump light
interval of 4.5 THz. FIG. 25 shows a gain profile obtained when the
pump light intervals are selected to 2.5 THz and 4.5 THz,
respectively and three wavelengths are used. In comparison with
FIG. 24, the valley of gain becomes shallower. Since the frequency
interval of 2.5 THz used here corresponds to the wavelength
interval of about 18 nm, also in this case, the wavelength interval
is included in the range from 6 nm to 35 nm. However, considering
conversely, it can be said that, even when the wavelength interval
is included in the range, if the wavelength interval is not set
properly, flat gain cannot be obtained.
By the way, in designing the conventional WDM optical amplifiers,
the object was to reduce the gain flatness as small as possible,
and an optical amplifier in which all of optical signals are
subjected to the same gain was ideal. When the number, power and
band width of the used signals are small, such design concept is
adequate. However, as the used band of the optical signal is
increased, there arose a problem regarding Raman amplifying effect
between optical signals. As disclosed in journal (for example, S.
Bigo et al, "IEEE Photonics Technology Letters", pp. 671-673,
1999), in this phenomenon, WDM signals which were set to have same
powers upon incident on a transmission line tend to include linear
tilt in which power becomes small at a short wavelength side and
great as a long wavelength side after transmission. Such tilt is
determined by various factors such as the number, power and band
width of the optical signals, property of a fiber constituting the
transmission line and a transmission distance. As means for coping
with this problem, there has been proposed an tilt compensator (T.
Naito OAA'99, WC5) for attenuating the long wavelength side signal
by using a loss medium having wavelength dependency and a method
(M. Takeda et al, OAA'99, ThA3) for compensating tilt by give
relatively great gain to the short wavelength side signal by using
wavelength dependency of Raman gain. Since the former method for
giving the loss has disadvantage due to noise, the later method is
more excellent. However, in the paper written by Takeda et al,
since the tilt of Raman gain is not linear, the gain flatness after
compensating the tilt is relatively great (more than 1 dB).
Similar to the WDM system using only the above-mentioned EDFA, also
in the WDM system using the Raman amplifier, when the WDM system
for C-band is introduced, it is desirable that the system is
designed so that the WDM system for L-band can be added while
maintaining the function of the equipment.
In a Raman amplifiers using wavelength multiplexed pumping, when it
is desired to expand the gain band, like band extension from C band
to C+L band, it is necessary that, while utilizing all of pumping
wavelengths which were used before expansion, after the expansion,
the amplifier can be operated for the C+L band. That is to say, it
must be designed so that, by adding new pumping wavelength to the
pumping wavelength used for the C-band, the amplifier can be
operated for the C+L band. In this case, it is necessary that
wavelength arrangement for flattening the gain in the C-band and
the C+L band can be commonly used.
Since the gain deviation is proportional to the magnitude of the
peak gain, if the gain is great, the pump light interval must be
set small. Further, as mentioned above, in a case where the pump
lights are equidistantly arranged, even when the wavelength
interval is smaller than 35 nm, the gain deviation may not be
reduced sufficiently. Also in this case, it is necessary to use
narrower wavelength interval. Although the gain deviation can be
reduced by reducing the pump light interval in principle, due to
problems regarding a wave combining technique and cost,
practically, the pump light interval has a lower limit. In Japanese
Patent Application Nos. 10-208450 (1998) and 11-34833 (1999), the
lower limit is determined to 6 nm on the basis of the wave
combining technique.
However, in the above-mentioned Japanese Patent Application Nos.
10-208450 (1998) and 11-34833 (1999), although the fact that the
interval between two adjacent pumping wavelengths is preferably
within a range from 6 nm to 35 nm is disclosed, adequate
information regarding detailed design values is not disclosed.
Further, in the design described in a published paper (Y. Emori et
al, OFC'99 PD19), the gain deviation is 1 dB, and this technique
cannot be applied when smaller gain flatness is required.
SUMMARY OF THE INVENTION
In the present invention, a method for selecting wavelength
disposition in a Raman amplifier using three or more pumping
wavelengths is disclosed, and a primary object of the present
invention is to provide a Raman amplifier having good gain
flatness, and further, an object of the present invention is to
provide a Raman amplifier in which gain deviation becomes about 0.1
dB for a peak value of Raman gain of 10 dB.
Further, another object of the present invention is to provide a
Raman amplifier suitable for compensating Raman effect between
signals which arises a problem during wide band WDM
transmission.
In the present invention, it is also considered to provide a Raman
amplifier in which, when a gain band is expanded by adding new pump
light, gain and gain flatness after expansion are not deteriorated
considerably in comparison with before expansion.
The Inventors investigated the gain profiles of Raman amplifiers
utilizing wavelength multiplexed pumping and analyzed wavelength
disposition for flattening the gain. The principle is as
follows.
The gain profile of the Raman amplifier utilizing wavelength
multiplexed pumping is obtained by the superposition of gains
generated by respective pumping wavelengths. Accordingly, a
combination of gain tilts which are cancelled with each other is
one of factors for obtaining flat gain. That is to say, a gain
property having good wavelength flatness can be obtained by
combining a rightwardly and downwardly extending curve (negative
gain tilt) in which the gain is decreased from the short wavelength
side toward the long wavelength side and a rightwardly and upwardly
extending curve (positive gain tilt) in which the gain is increased
from the short wavelength side toward the long wavelength side.
When the number of pumping wavelengths is two, the tilt at a longer
wavelength side from gain peak of a gain curve obtained by the
shorter wavelength pump is combined with the tilt at a shorter
wavelength side from gain peak of a gain curve obtained by the
longer wavelength pump. As apparent from FIG. 21, the Raman gain
curve of one wavelength pumping which is a basic element for
superposition has two gain peaks rather than one, and, in the
C-band, the first peak at the end of positive gain tilt located at
1550 nm and the second peak at the end of negative gain tilt is
located at 1560 nm. Further, in the L-band, the first peak at the
end of positive gain tilt is located at 1595 nm and the second peak
at the end of negative gain tilt is located at 1605 nm. Thus, in
any cases, two pumping wavelenghts must be spaced apart from each
other by 10 nm or more. In the curve A shown FIG. 21, the center
wavelength of the pumping wavelength is 1450 nm, and, in the curve
B, the center wavelength of the pumping wavelenght is 1490 nm.
FIG. 26 shows an example of the gain profile of the Raman amplifier
designed for the C-band and the following Table 3 shows pumping
wavelengths used therefor. A supposed fiber is a normal single mode
fiber, and the gain band is designed to cover 1530 nm to 1565 nm.
The gain profile of the Raman amplifier utilizing the wavelength
multiplexed pumping is obtained by the superposition of gains
generated by respective pumping wavelengths. In FIG. 26 the
allocation of gain magnitude caused by each pump wavelength is
optimized to minimize the flatness of the gain obtained by
addition. The number of pumping wavelengths is appropriately
selected in accordance with the desired gain flatness. Similarly,
FIG. 27 shows an example of the gain profile of the Raman amplifier
designed for the L-band and the following Table 4 shows pumping
wavelengths used therefor.
TABLE 3 2 wavelength pumping 3 wavelength pumping 4 wavelength
pumping 1426 1424 1423 1453 1435 1430 1460 1438 (Unit: nm) 1462
TABLE 4 2 wavelength pumping 3 wavelength pumping 4 wavelength
pumping 1464 1463 1462 1493 1475 1470 1500 1478 (Unit: nm) 1501
When the number of pumping wavelengths is two, the gain band width
can be widened by increasing the interval between the wavelengths.
However, if the interval is too wide, the valley of gain will be
created in the band. Accordingly, the gain flatness and the gain
band have a relationship of "trade-off." In FIGS. 26 and 27, the
wavelength intervals of pump lights are determined to obtain the
optimized results in pre-determined gain bands, and such intervals
are 27 nm and 29 nm, respectively (refer to the Tables 3 and
4).
In the Raman gain curve of one wavelength pumping which is a basic
element for superposition, as shown in FIG. 21, the gain tilt at
the longer wavelength side from the gain peak is more steep than
the gain tilt at the shorter wavelength, and the band in which the
tilt can be utilized is narrow. In order to widen the band by using
a more gentle negative gain tilt, the negative gain tilt must be
formed by using a plurality of pumping wavelengths.
When the gain curve having the negative gain tilt is formed by
using three or more pumping wavelengths, similar to two pumping
wavelengths, the pumping wavelengths for forming the negative gain
tilt must be spaced apart from the pumping wavelengths for forming
the positive gain tilt by 10 nm or more. However, since the pump
light for forming the negative gain tilt is constituted by the
plurality of wavelengths, the longest pumping wavelength among them
is spaced apart from the pumping wavelength for forming the
positive gain tilt by 10 nm or more. This corresponds to the
interval between 1435 nm and 1460 nm in three (3) wavelengths
pumping and the interval between 1438 nm and 1462 nm in four (4)
wavelengths pumping in the Table 3. Further, this corresponds to
the interval between 1475 nm and 1500 nm in three (3) wavelengths
pumping and the interval between 1478 nm and 1501 nm in four (4)
wavelengths pumping in the Table 4.
In a case where three or more pumping wavelengths are used, when
the intervals between the pumping wavelengths is approximately
equidistant, ripple in the negative gain tilt generated by the
combination becomes small. When the flatness is obtained by
combination with the positive gain tilt, such ripple determines the
final gain flatness. It is demonstrated by the four wavelength
pumping case in FIG. 26 and FIG. 27. As shown in the Tables 3 and
4, the optimized wavelengths are 1423 nm, 1430 nm, 1438 nm and 1462
nm, 1470 nm, 1478 nm, which provide approximately equidistant
disposition.
FIGS. 28 to 30 show the performance of the Raman gain curves when
the pump light intervals are equidistant. FIG. 28 shows an example
that the peak gain is adjusted to 10 dB under a condition that the
gains generated by each pump lights are the same. From FIG. 28, it
can be seen that the narrower the pump light interval is, the
smaller the unevenness of the gain is. FIG. 29 shows an example
that the gains obtained by the respective pump lights are adjusted
to flatten the gain. Also in this case, similar to FIG. 28, the
narrower the pump light interval is, the smaller the unevenness of
the gain is. Further, it can be seen that undulation of the gain
curve in FIG. 28 determines the maximum gain deviation in FIG. 29.
Thus, in order to bring the gain deviation to about 0.1 dB, the
pump light interval of 2 THz is too wide, but 1 THz is
adequate.
FIG. 30 shows the pattern when the multiplexed amount in number is
changed while maintaining the pump light interval in 1 THz. As can
be seen from a gain curve of 1ch pumping, in case of a silica-based
fiber, a smooth curve having no unevenness is obtained at the
shorter wavelength side from the gain peak, however, there are
three relatively prominent unevenness at the longer wavelength
side, which is a factor for determining the limitation of the
flatness. The unevenness is decreased as the multiplexed amount in
number is increased. For example, referring to the gain curve of
1ch, although this curve has protrusion of about 1 dB in the
vicinity of 187 THz, as the multiplexed amount in number is
increased, such protrusion gradually becomes smaller. The reason is
that, since the peak gains are set to be the same, as the
multiplexed amount in number is increased, the gain per one wave is
decreased to reduce the magnitude of the protrusion itself and the
unevenness of the same shape (concavities and convexities) are
added with slight equidistant intervals. That is to say, the
convexities of the gain curve of a certain pumping wavelength are
added to the concavities of the gain curve of another pumping
wavelength to reduce the entire unevenness. The value "about 1 THz"
specified in claims 2 to 4 is based on this principle and is also
based on the fact that frequency difference between the protrusion
in the vicinity of 187 THz and the immediately adjacent recess in
the vicinity of 188 THz is about 1 THz in the gain curve of 1ch
pumping shown in FIG. 30. Accordingly, depending upon the fiber
used, the gain curve of 1ch pumping may be slightly changed and,
thus, the value specified in claims 2 to 4 as "about 1 THz" may
also be changed. In any case, in order to reduce the gain
deviation, the unevenness of the gain curves to be added to each
other must be cancelled with each other.
Since the limit of the gain deviation is determined by the
undulation and unevenness of the each gain curves to be overlapped
or combined, it is considered that a flat gain profile having small
gain deviation can be obtained by combining the gain curves having
small unevenness. Accordingly, this can be achieved by combining
the gain curve generated by the pump light multiplexed with
interval of about 1 THz with the gain curve generated by the pump
light located at the longer wavelength side than the former pump
light. In this case, in view of expansion of the band, it is
desirable that the peaks of two gain curves are spaced apart from
each other moderately.
While the effects as mentioned above was explained for the purpose
of reducing the gain flatness, by reducing the gain of the pump
light at the long wavelength side, a gain profile in which gain is
linearly reduced from the short wavelength side toward the long
wavelength side can be achieved. When this is combined with the
tilt of optical signal level created by the Raman effect between
the optical signals, the level of the optical signal can be
flattened. Since any tilt can be realized by adjusting the
distribution of gain between the short wavelength side and the long
wavelength side, any Raman tilt can be compensated for.
When the gain bands of the C-band and L-band are tried to be
expanded, it is considered that simultaneous use of the pumping
wavelength for C-band and the pumping wavelength for L-band is
optimum.
However, when both the pump lights for C-band and L-band shown in
FIGS. 26 and 27 are simultaneously used, gain profiles such as
shown in FIG. 31 are obtained even if the gain allocation is
optimized so as to be flatten. The following Table 5 pumping
wavelengths used in this case. In this case, if the gain flatness
is tried to be equal to those shown in FIGS. 26 and 27 in the
entire area of C+L as shown in FIG. 31, a big recess is generated
in the C-band, thereby more worsening the gain flatness than before
expansion.
TABLE 5 2 wave for C-band + 3 wave for C-band + 4 wave for C-band +
2 wave for L-band 3 wave for L-band 4 wave for L-band 1426 1424
1423 1453 1435 1430 1464 1460 1438 1493 1463 1462 1475 1470 1500
1478 (Unit: nm) 1501
For the same reasons as mentioned above, also when the operation of
good gain flatness is effected in the C+L band, it is necessary
that the negative gain tilt is formed by using the plurality of
equidistant pumping wavelenghts and the positive gain tilt is
formed by using the pump light located at the longer wavelenght
side from the longest pumping wavelength by 10 nm or more. However,
since the pumping wavelengths used in FIG. 31 are obtained only by
simultaneously using the pumping wavelengths of FIGS. 26 and 27,
the requirement "approximately equidistant disposition" cannot be
satisfied in the pumping wavelength band for C-band. The Table 5
shows the pumping approximately used in FIG. 31. According to this,
in order to obtain the approximately equidistant disposition at the
short wavelength side, it can be seen that the pumping wavelength
is insufficient in the pumping wavelength band for C-band. It was
found that such insufficient amount becomes a factor for generating
the recess in gain shown in FIG. 31.
From the above results, it was analyzed that means required for
achieving the above object are as follows.
According to a means of the present invention, in a Raman amplifier
using three or more pumping wavelengths, when the pumping
wavelengths are divided into a short wavelength side group and a
long wavelength side group at the boundary of the pumping
wavelength having the longest interval between the adjacent
wavelengths, the short wavelength side group includes two or more
pumping wavelengths having intervals therebetween which are
substantially equidistant, and the long wavelength side group is
constituted by two or less pumping wavelengths.
According to another means of the present invention, when a
shortest pumping wavelength is defined as a first channel and
pumping wavelengths which are spaced apart from each other by about
1 THz from the shortest pumping wavelength toward a long wavelength
side are defined as second to n-th channels, respectively, pump
lights having wavelengths corresponding to the first to n-th
channels are multiplexed, and pump light having a wavelength spaced
apart from the n-th channel by 2 THz or more toward the long
wavelength side is further combined, and resultant pump light is
used as pump light for a Raman amplifier. Further, when the
shortest pumping wavelength is defined as the first channel and
pumping wavelengths which are spaced apart from each other by about
1 THz from the shortest pumping wavelength toward the long
wavelength side are defined as the second to n-th channels,
respectively, all of the wavelengths corresponding to the channels
other than (n-1)th and (n-2)th channels are combined with each
other, resultant pump light is used for the Raman amplifier.
Alternatively, all of the wavelengths corresponding to the channels
other than (n-2)th and (n-3)th channels are combined with each
other, resultant pump light is used for the Raman amplifier.
According to still another means of the present invention, in a
Raman amplifier for expanding a gain wavelength band, there are
provided two or more pumping wavelengths before expansion, and two
or more pumping wavelengths are added for expanding the gain
wavelength band, at least one of the pumping wavelengths to be
added is differentiated from the pumping wavelengths used before
expansion, and at least one of the differentiated pumping
wavelengths is positioned within the pumping wavelengths bands used
before expansion.
According to still another means of the present invention, in a
Raman amplifier for expanding a gain wavelength band, there are
provided two or more pumping wavelengths before expansion, and two
or more pumping wavelengths are added for expanding the gain
wavelength band, at least one of the pumping wavelengths to be
added is differentiated from the pumping wavelengths used before
expansion, and at least one of the differentiated pumping
wavelengths is positioned within a pumping wavelength band having
insufficient gain among the pumping wavelengths bands used before
expansion.
According to still another means of the present invention, in a
Raman amplifier for expanding a gain wavelength band, there are
provided two or more pumping wavelengths before expansion, and one
or more pumping wavelength is added to the pumping wavelengths
bands before expansion so that, by the addition, the pumping
wavelengths within the pumping wavelengths bands before expansion
are spaced apart from each other equidistantly or substantially
equidistantly.
According to a Raman amplifier of the present invention, when a
C-band and an L-band are simultaneously amplified by simultaneously
using two or more pumping wavelengths for amplifying the C-band and
two or more pumping wavelengths for amplifying the L-band, one or
more pumping wavelength different from the pumping wavelengths for
the C-band used before expansion is added to bands of the pumping
wavelengths for the C-band.
According to another Raman amplifier of the present invention, when
a C-band and an L-band are simultaneously amplified by
simultaneously using two or more pumping wavelengths for amplifying
the C-band and two or more pumping wavelengths for amplifying the
L-band, one or more pumping wavelength different from the pumping
wavelengths for the C-band used before expansion is added to a band
of a wavelength having insufficient gain among bands of the pumping
wavelengths for the C-band.
According to still another Raman amplifier of the present
invention, when a C-band and an L-band are simultaneously amplified
by simultaneously using two or more pumping wavelengths for
amplifying the C-band and two or more pumping wavelengths for
amplifying the L-band, one or more pumping wavelength different
from the pumping wavelengths for the C-band used before expansion
is added to bands of the pumping wavelengths for the C-band so
that, by the addition, the pumping wavelengths within the bands of
the pumping wavelengths before expansion are spaced apart from each
other equidistantly or substantially equidistantly.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an explanatory view showing a Raman amplifier according
to a first embodiment of the present invention;
FIG. 2 is a view showing Raman gain profiles obtained when a pump
source shown in FIG. 1 is used;
FIG. 3 is an enlarged view showing a total gain shown in FIG.
2;
FIG. 4 is a view showing Raman gain profiles obtained when a
wavelength of a sixth channel is spaced apart from a fifth channel
by 2.5 THz toward a long wavelength side, in the Raman amplifier
shown in FIG. 1;
FIG. 5 is an enlarged view showing a total gain shown in FIG.
4;
FIG. 6 is an explanatory view showing a Raman amplifier according
to a second embodiment of the present invention;
FIG. 7 is a view showing Raman gain profiles obtained when a pump
source shown in FIG. 6 is used;
FIG. 8 is an enlarged view showing a total gain shown in FIG.
7;
FIG. 9 is an explanatory view showing a Raman amplifier according
to a third embodiment of the present invention;
FIG. 10 is a view showing Raman gain profiles obtained when a pump
source shown in FIG. 9 is used;
FIG. 11 is an enlarged view showing a total gain shown in FIG.
10;
FIG. 12 is a view showing Raman gain profiles obtained when eleven
channels are used among thirteen channels located equidistantly by
1 THz interval from 211 THz to 199 THz and when pump lights other
than 201 THz and 200 THz are used;
FIG. 13 is an enlarged view showing a total gain shown in FIG.
12;
FIG. 14 is a view showing Raman gain profiles obtained when eleven
channels are used among thirteen channels located equidistantly by
1 THz interval from 211 THz to 199 THz and when pump lights other
than 202 THz and 201 THz are used;
FIG. 15 is an enlarged view showing a total gain of gains shown in
FIG. 14;
FIG. 16 is a view showing Raman gain profiles obtained when the
same pump source as those in FIG. 12 are used;
FIG. 17 is an explanatory view showing gain properties when pump
light before expansion has two wavelengths;
FIG. 18 is an explanatory view showing gain properties when pump
light before expansion has three wavelengths;
FIG. 19 is an explanatory view showing gain properties when pump
light before expansion has four wavelengths;
FIG. 20 is an explanatory view for a Raman amplifier;
FIG. 21 is an explanatory view showing gain properties when a
C-band and an L-band are pumped by a single wavelength,
respectively;
FIG. 22 is a view showing a relationship between size of gain and a
gain band width;
FIGS. 23A and 23B are views showing Raman gain profiles obtained
when the pump light intervals are selected to 4.5 THz and 5 THz,
respectively and DSF is used as an amplifying fiber;
FIG. 24 is a view showing Raman gain profiles obtained when three
wavelengths with pump light interval of 4.5 THz are used;
FIG. 25 is a view showing Raman gain profiles obtained when three
wavelengths with pump light intervals of 2.5 THz and 4.5 THz are
used;
FIG. 26 is an explanatory view showing wave forms in an example
that the Raman amplifying method of the present invention is
applied to a C-band;
FIG. 27 is an explanatory view showing wave forms in an example
that the Raman amplifying method of the present invention is
applied to an L-band;
FIG. 28 is a view showing performance of Raman gain curves when the
pump light intervals are equidistant and when the peak gain is
adjusted to be 10 dB under a condition that gains generated by
respective pump lights are the same;
FIG. 29 is a view showing performance of Raman gain curves when the
pump light intervals are equidistant and when gains of respective
pump lights are adjusted to flatten the gains;
FIG. 30 is a view showing performance of Raman gain curves when the
pump light intervals are equidistant and when the multiplexing
number is changed with pump light interval of 1 THz; and
FIG. 31 is an explanatory view showing gain properties obtained
when the Raman amplifying method of the present invention is
applied to a C+L band.
BEST MODES FOR CARRYING OUT THE PRESENT INVENTION
In embodiments described hereinbelow, examples that a first channel
is 211 THz are shown. The reason is that a wavelength band greater
than 1530 nm (corresponding to frequency smaller than about 196
THz) which has been used in present WDM systems is supposed as an
amplifying band. Accordingly, if a so-called L-band greater than
1580 nm (corresponding to frequency smaller than about 190 THz) is
supposed as the amplifying band, since a pumping band may be
shifted by 6 THz, a first channel may be 205 THz. Regarding the
other amplifying bands, the first channel can be determined in a
similar manner.
FIG. 1 shows a first embodiment of the present invention which
corresponds to claim 2. Frequency of the first channel is 211 THz
(corresponding to a wavelength of 1420.8 nm), and frequencies of
second to fifth channels are equidistantly distributed between 210
THz (corresponding to a wavelength of 1427.6 nm) and 207 THz
(corresponding to a wavelength of 1448.3 nm) with interval of 1
THz. A wavelength multiplexed pump source for a Raman amplification
is obtained by combining pump light (frequency of 205 THz,
wavelength of 1462.4 nm) having a wavelength spaced apart from the
fifth channel by 2 THz toward a long wavelength side with the
above-mentioned channels. For each of pump sources 10 of respective
wavelengths shown in FIG. 1, pumping power obtained by combining
outputs of semi-conductor lasers of Fabry-Perot type
wavelength-stabilized by fiber Bragg grating (FBG) by means of a
polarization beam coupler (PBC). Polarization beam coupling is an
action for increasing pump power of the each wavelength and at the
same time for reducing polarization dependence of Raman gain. If
the pump power outputted from the single laser is sufficient, after
the laser output was depolarized, it may be connected to a
wavelength multiplexer. A multiplexer 20 of Mach-Zehnder
interferometer type shown in FIG. 1 serves to multiplex or combine
pump lights of a plurality of wavelength having adjacent
wavelengths with constant frequency interval. A dielectric filter
30 shown in FIG. 1 serves to combine two relatively wide wavelength
bands and can combine a wavelength greater than a certain
wavelength with a wavelength smaller than the certain wavelength.
In the illustrated embodiment, a filter 30 capable of combining
frequency higher than 207 THz (wavelength shorter than 1448.3 nm)
with frequency lower than 205 THz (wavelength longer than 1462.4
nm) is used. In FIG. 1, the pump light combined by the dielectric
filter 30 is sent to a dielectric filter 50 through an isolator 40
to amplify a WDM signal in an optical fiber.
FIG. 2 shows Raman gain profiles obtained when the pump light
sources shown in FIG. 1 are used. A curve A represents total gain,
a curve B represents a sum of gains obtained by the pump lights of
the first to fifth channels, a curve C represents gain of the sixth
channel, and fine line curves represent gains obtained by the
pumping wavelengths of the first to fifth channels. As will be
described later regarding effects, by multiplexing the pump lights
at the short wavelength side with interval of 1 THz, a flat curve
with negative gain tilt can be formed, and, by adding a positive
gain tilt obtained from the pump lights at the long wavelength side
to this negative gain tilt, the total Raman gain is flattened. From
FIG. 2, it can be seen that unevenness in the plural gain curves is
well cancelled by using the interval of 1 THz. FIG. 3 is an
enlarged view of the total gain. A property in which peak gain is
10 dB and a gain band is about 196 THz (wavelength: 1529.6 nm) to
193 THz (wavelength: 1553.3 nm) and gain deviation is about 0.1 dB
can be realized.
FIG. 4 shows gain profiles obtained when the wavelength of the
sixth channel is a wavelength (frequency: 204.5 THz, wavelength:
1465.5 nm) spaced apart from the fifth channel by 2.5 THz toward
the long wavelength side in FIG. 1. Similar to FIG. 2, a curve A
represents total gain, a curve B represents a sum of gains obtained
by the pump lights of the first to fifth channels, a curve C
represents gain of the sixth channel, and fine line curves
represent gains obtained by the pumping wavelengths of the first to
fifth channels. Also in this case, the total Raman gain is
flattened by adding a negative gain tilt obtained from the pump
lights at the short wavelength side to a positive gain tilt
obtained from the pump lights at the long wavelength side. FIG. 5
is an enlarged view of the total gain. A property in which peak
gain is 10 dB and a gain band is about 196 THz (wavelength: 1529.6
nm) to 192 THz (wavelength: 1561.4 nm) and gain deviation is about
0.1 dB can be realized. A gain band is wider than that in FIG. 3,
and a recess in the gain at an intermediate area of the band is
slightly greater than that in FIG. 3. The reason is that the
interval between the fifth channel and the sixth channel is
wider.
FIG. 6 shows a second embodiment of the present invention which
corresponds to claims 2 and 3. Frequency of the first channel is
211 THz (corresponding to a wavelength of 1420.8 nm), and
frequencies of second to eighth channels are equidistantly
distributed between 210 THz (corresponding to a wavelength of
1427.6 nm) and 204 THz (corresponding to a wavelength of 1469.6 nm)
with interval of 1 THz. The total number of channels is eight (8),
and a pump source is constituted by using six wavelengths other
than sixth and seventh channels. As explained in connection with
the first embodiment, the pump lights of respective channels may be
selected on demand. For each of pump light sources 10 of respective
wavelengths shown in FIG. 6, pump power obtained by combining
outputs of semi-conductor lasers of Fabry-Perot type
wavelength-stabilized by fiber Bragg grating (FBG) by means of a
polarization beam coupler (PBC). Polarization beam coupling is an
action for increasing pump power of the each wavelength and at the
same time for reducing polarization beam dependency of Raman gain.
If the pump power outputted from the single laser is sufficient,
after the laser output was non-polarized, it may be connected to a
wavelength multiplexer. A multiplexer 20 of Mach-Zehnder
interferometer type shown in FIG. 6 serves to multiplex or combine
pump lights of a plurality of wavelength having adjacent
wavelengths with constant frequency interval. A dielectric filter
30 shown in FIG. 6 serves to combine two relatively wide wavelength
bands and can combine a wavelength greater than a certain
wavelength with a wavelength smaller than the certain wavelength.
In FIG. 6, the pump light outputted from the multiplexer 20 of
Mach-Zehnder interferometer type is sent to a dielectric filter 50
through an isolator 40 to amplify a WDM signal in an optical fiber.
FIG. 7 shows Raman gain profiles obtained when the pump sources
shown in FIG. 6 are used. A curve A represents total gain, a curve
B represents a sum of gains obtained by the pump lights of the
first to fifth channels, a curve C represents gain of the eighth
channel, and fine line curves represent gains obtained by the
pumping wavelengths of the first to fifth channels. Also in this
case, by adding a positive gain tilt obtained from the pump lights
at the long wavelength side to a negative gain tilt obtained from
the pump lights at the short wavelength side, the total Raman gain
is flattened. FIG. 8 is an enlarged view of the total gain. A
property in which peak gain is 10 dB and a gain band is about 196
THz (wavelength: 1529.6 nm) to 191 THz (wavelength: 1569.6 nm) and
gain deviation is about 0.1 dB can be realized. Comparing with FIG.
5 and FIG. 3, the gain band is further widened. The reason is that
the longest pumping wavelength is set as a longer wavelength.
FIG. 9 shows a third embodiment of the present invention which
corresponds to claims 2 and 4. Similar to the second embodiment,
frequency of the first channel is 211 THz (corresponding to a
wavelength of 1420.8 nm), and frequencies of second to eighth
channels are equidistantly distributed between 210 THz
(corresponding to a wavelength of 1427.6 nm) and 204 THz
(corresponding to a wavelength of 1469.6 nm) with interval of 1
THz. The total number of channels is eight (8), and a pump source
is constituted by using six wavelengths other than fifth and sixth
channels. For each of pump sources 10 of respective wavelengths
shown in FIG. 9, pump power obtained by combining outputs of
semi-conductor lasers of Fabry-Perot type wavelength-stabilized by
fiber Bragg grating (FBG) by means of a polarization beam coupler
(PBC). Polarization beam coupling is an action for increasing pump
power of the each wavelength and at the same time for reducing
polarization beam dependency of Raman gain. If the pump power
outputted from the single laser is sufficient, after the laser
output was non-polarized, it may be connected to a wavelength
multiplexer. A multiplexer 20 of Mach-Zehnder interferometer type
shown in FIG. 9 serves to multiplex or combine pump lights of a
plurality of wavelength having adjacent wavelengths with constant
frequency interval. A dielectric filter 30 shown in FIG. 9 serves
to combine two relatively wide wavelength bands and can combine a
wavelength greater than a certain wavelength with a wavelength
smaller than the certain wavelength. In FIG. 9, the pump light
outputted from the multiplexer 20 of Mach-Zehnder interferometer
type is sent to a dielectric filter 50 through an isolator 40 to
amplify a WDM signal in an optical fiber. FIG. 10 shows Raman gain
profiles obtained when the pump sources shown in FIG. 9 are used. A
curve A represents total gain, a curve B represents a sum of gains
obtained by the pump lights of the first to fourth channels, a
curve C represents a sum of gains of the seventh and eighth
channels, and fine line curves represent gains obtained by the
respective pumping wavelengths. Also in this case, by adding a
positive gain tilt obtained from the pump lights at the long
wavelength side to a negative gain tilt obtained from the pump
lights at the short wavelength side, the total Raman gain is
flattened. FIG. 11 is an enlarged view of the total gain. A
property in which peak gain is 10 dB and a gain band is about 196
THz (wavelength: 1529.6 nm) to 191 THz (wavelength: 1569.6 nm) and
gain deviation is about 0.1 dB can be realized. Here, a difference
in magnitude of the gains obtained by the pumping wavelengths in
the second embodiment and the third embodiment should be noticed.
In FIG. 7, there is a channel having about 8 dB at the maximum;
whereas, in FIG. 10, the maximum is about 5 dB. The reason is that,
in the second embodiment, the gain shown by the curve C at the long
wavelength side is formed by the gain of the single channel,
whereas, in the third embodiment, the gain at the long wavelength
side is formed by the sum of the gains of two channels. This means
that the maximum value of the pump power required for each wave can
be reduced, and it is very effective in view of the practical
use.
FIGS. 12 to 15 show gain profiles obtained when eleven channels are
used among thirteen channels spaced apart from each other with
interval of 1 THz from 211 THz (wavelength: 1420.8 nm) to 199 THz
(wavelength: 1506.5 nm). In FIG. 12, a construction specified in
claim 3 is used and pump lights other than 201 THz and 200 THz are
used. A curve A represents total gain, a curve B represents a sum
of gains obtained by pump lights of first to tenth channels, a
curve C represents gain of thirteenth channel, and fine line curves
represent gains obtained by the pumping wavelengths of the first to
tenth channels. Also in this case, by adding a positive gain tilt
obtained from the pump lights at the long wavelength side to a
negative gain tilt obtained from the pump lights at the short
wavelength side, the total Raman gain is flattened. FIG. 13 is an
enlarged view of the total gain. A property in which peak gain is
10 dB and a gain band is about 196 THz (wavelength: 1529.6 nm) to
186 THz (wavelength: 1611.8 nm) and gain deviation is about 0.1 dB
can be realized.
In FIG. 14, a construction specified in claim 4 is used and pump
lights other than 202 THz and 201 THz are used. A curve A
represents total gain, a curve B represents a sum of gains obtained
by pump lights of first to ninth channels, a curve C represents a
sum of gains of twelfth and thirteenth channel, and fine line
curves represent gains obtained by the pumping wavelengths. Also in
this case, by adding a positive gain tilt obtained from the pump
lights at the long wavelength side to a negative gain tilt obtained
from the pump lights at the short wavelength side, the total Raman
gain is flattened. FIG. 15 is an enlarged view of the total gain. A
property in which peak gain is 10 dB and a gain band is about 196
THz (wavelength: 1529.6 nm) to 186 THz (wavelength: 1611.8 nm) and
gain deviation is about 0.1 dB can be realized. Further, as can be
seen from comparison between FIG. 12 and FIG. 14, since the gain
shown by the curve C at the long wavelength side is formed by the
gain of the single channel in the second embodiment, whereas, in
the third embodiment, the gain at the long wavelength side is
formed by the sum of the gains of two channels, the maximum value
of the gain required for each wave is smaller in FIG. 14 than in
FIG. 12. This means that the maximum value of the pump power
required for each wave can be reduced, and it is very effective in
view of the practical use.
FIG. 16 shows gain profiles obtained when the same pump sources as
those in FIG. 12 are used, and, linear negative gain tilts are
realized by reducing the gains at the long wavelength side. By
using such a Raman amplifier, positive gain tilt due to the Raman
effect between the optical signals described in the paper written
by Bigo et al is compensated, and the WDM signal can be maintained
to the flat level in the optical amplifying relay system. For
example, in FIG. 2 of the paper written by Bigo et al, gain tilt of
2.3 dB is generated at 25 nm, and, by adding reverse tilt for
causing reduction of 7.4 dB (converted at 80 nm) to the former
tilt, the tilt of optical signal level due to the Raman effect
between the optical signals can be cancelled. In FIG. 16, although
the gain tilts for causing reduction of 3 dB, 5 dB and 7 dB at 80
nm are shown, since they are 2.2 dB, 1.6 dB and 0.9 dB converted at
25 nm, even under the condition shown in FIG. 3 of the
above-mentioned paper, it is considered that the gain tilt can be
compensated for.
EXAMPLE 1
Now, the Raman amplifier according to the present invention will be
explained in connection with an example that the band is expanded
to C+L band by combining the C band and the L band. FIG. 17 shows
an example that the pump light has two wavelengths before
expansion, and there are pumping wavelengths before and after
expansion, as shown in the following Table 6. By selecting one of
pumping wavelengths added for the expansion to 1439 nm, at least
one of the pump lights added for the expansion is located within
the band of the pump lights (1426 nm to 1453 nm) before expansion.
Due to the presence of such pump light, the expansion can be
realized while maintaining the gain flatness.
TABLE 6 Pumping wavelength [nm] After expansion Before expansion
Addition 1426 .smallcircle. .smallcircle. 1439 .smallcircle.
.smallcircle. 1453 .smallcircle. .smallcircle. 1464 .smallcircle.
.smallcircle. 1493 .smallcircle. .smallcircle.
FIG. 18 shows an example that the pump light has three wavelengths
before expansion, and there are pumping wavelengths before and
after expansion, as shown in the following Table 7. By selecting
one of pumping wavelenghts added for the expansion to 1446 nm, at
least one of the pump lights added for the expansion is located
within the band of the pump lights (1424 nm to 1460 nm) before
expansion. Due to the presence of such pump light, the expansion
can be realized while maintaining the gain flatness.
TABLE 7 Pumping wavelength [nm] After expansion Before expansion
Addition 1424 .smallcircle. .smallcircle. 1435 .smallcircle.
.smallcircle. 1446 .smallcircle. .smallcircle. 1460 .smallcircle.
.smallcircle. 1463 .smallcircle. .smallcircle. 1475 .smallcircle.
.smallcircle. 1500 .smallcircle. .smallcircle.
FIG. 19 shows an example that the pump light has four wavelengths
before expansion, and there are pumping wavelengths before and
after expansion, as shown in the following Table 8. By selecting
two of pumping wavelenghts added for the expansion to 1445 nm and
1453 nm, at least one of the pump lights added for the expansion is
located within the band of the pump lights (1423 nm to 1462 nm)
before expansion. Due to the presence of such pump light, the
expansion can be realized while maintaining the gain flatness.
Incidentally, in this example, since the pumping wavelength of 1462
nm is used for respective independent designs for C band and L
band, addition is not required for expansion.
TABLE 8 Pumping wavelength [nm] After expansion Before expansion
Addition 1423 .smallcircle. .smallcircle. 1430 .smallcircle.
.smallcircle. 1438 .smallcircle. .smallcircle. 1445 .smallcircle.
.smallcircle. 1453 .smallcircle. .smallcircle. 1462 .smallcircle.
.smallcircle. 1470 .smallcircle. .smallcircle. 1478 .smallcircle.
.smallcircle. 1501 .smallcircle. .smallcircle.
EXAMPLE 2
This example 2 shows an example that the pump light has two
wavelenghts before expansion and the wavelength of the pump light
before expansion is greater than that in the example 1. In this
example, the gain band for the C band is designed to be 1535 nm to
1570 nm. The pumping wavelenghts before and after expansion are as
shown in the Table 9. By selecting one of pumping wavelengths added
for the expansion to 1444 nm, at least one the pump lights added
for the expansion is located within the band of the pump lights
(1430 nm to 1457 nm) before expansion. Due to the presence of such
pump light, the expansion can be realized while maintaining the
gain flatness.
TABLE 9 Pumping wavelength [nm] After expansion Before expansion
Addition 1430 .smallcircle. .smallcircle. 1444 .smallcircle.
.smallcircle. 1457 .smallcircle. .smallcircle. 1464 .smallcircle.
.smallcircle. 1493 .smallcircle. .smallcircle.
EXAMPLE 3
This example 3 shows an example that the pump light has two
wavelengths before expansion and the wavelength of the pump light
before expansion is smaller than that in the example 1. In this
example, the gain band of the C band is designed to be 1525 nm to
1560 nm. The pumping wavelengths before and after expansion are as
shown in the following Table 10. By selecting one of pumping
wavelengths added for the expansion to 1438 nm, at least one of the
pump lights added for the expansion is located within the band of
the pump lights (1422 nm to 1450 nm) before expansion. Due to the
presence of such pump light, the expansion can be realized while
maintaining the gain flatness.
TABLE 10 Pumping wavelength [nm] After expansion Before expansion
Addition 1422 .smallcircle. .smallcircle. 1438 .smallcircle.
.smallcircle. 1450 .smallcircle. .smallcircle. 1464 .smallcircle.
.smallcircle. 1493 .smallcircle. .smallcircle.
INDUSTRIAL AVAILABLITY
According to a Raman amplifier specified in claim 1, in a Raman
amplifier using three or more pumping wavelengths, when the pumping
wavelengths are divided into the short wavelength side group and
the long wavelength side group at the boundary of the pumping
wavelength having the longest interval between the adjacent
wavelengths, since the short wavelength side group includes two or
more pumping wavelengths having intervals therebetween which are
substantially equidistant and the long wavelength side group is
constituted by two or less pumping wavelengths, the negative gain
tilt having wide band and less unevenness is formed by the short
wavelength side group and, by combining it with the positive gain
tilt formed by the long wavelength side group, a Raman amplifier
having a wide band and good gain flatness can be achieved.
In Raman amplifiers specified in claims 2 to 4, since an interval
of the pumping wavelength of the short wavelength side group is
about 1 THz, a Raman amplifier in which the gain deviation is about
0.1 dB with respect to the peak value of Raman gain of 10 dB can be
realized.
In a Raman amplifier specified in claim 5, when there are provided
two or more pumping wavelengths before gain band expansion, since
two or more new pumping wavelengths differentiated from the pumping
wavelengths used before expansion are added and at least one of the
pumping wavelengths to be added is differentiated from the pumping
wavelengths used before expansion and the added pumping wavelengths
are positioned in bands of the pumping wavelengths used before
expansion, the added bands are pumped to increase gains of such
bands, thereby flattening the gain in a wide band and expanding the
gain band.
In a Raman amplifier specified in claim 6, since at least one of
the pumping wavelengths to be added is differentiated from the
pumping wavelengths used before expansion and at least one of the
differentiated pumping wavelengths is positioned in a band of an
pumping wavelength having insufficient gain among bands of the
pumping wavelengths used before expansion, the pumping wavelength
insufficient band is pumped to increase gain of such a band,
thereby flattening the gain in a wide band and expanding the gain
band.
In a Raman amplifier specified in claim 1, since at least one of
the pumping wavelengths to be added is differentiated from the
pumping wavelengths used before expansion, and, by adding one or
more such pumping wavelengths, since the pumping wavelengths within
the bands of the pumping wavelengths before expansion are spaced
apart from each other equidistantly or substantially equidistantly,
the entire pumping wavelength bands are pumped, thereby flattening
the gain in a wide band and expanding the gain band.
In a Raman amplifier specified in claim 2, when a C-band and an
L-band are simultaneously amplified, since one or more pumping
wavelength different from the pumping wavelengths for the C-band
used before expansion is added to bands of the pumping wavelengths
for the C-band, the added band in the C-band is pumped to increase
gain of such a band, thereby flattening the gain in a wide band and
expanding the gain band.
In a Raman amplifier specified in claim 3, when a C-band and an
L-band are simultaneously amplified, since one or more pumping
wavelength different from the pumping wavelengths for the C-band
used before expansion is added to a band of a wavelength having
insufficient gain among bands of the pumping wavelengths for the
C-band, the pumping wavelength insufficient band in the C-band is
pumped to increase gain of such a band, thereby flattening the gain
in a wide band and expanding the gain band.
In a Raman amplifier specified in claim 4, when a C-band and an
L-band are simultaneously amplified, since one or more pumping
wavelength different from the pumping wavelengths for the C-band
used before expansion is added to bands of the pumping wavelengths
before expansion so that the pumping wavelengths within the bands
of the pumping wavelengths for the C-band before expansion are
spaced apart from each other equidistantly or substantially
equidistantly, the entire pumping wavelength bands for the C-band
are pumped, thereby flattening the gain in a wide band and
expanding the gain band.
* * * * *